1. Technical Field of the Invention
The present invention relates generally to wireless communication systems and, more particularly, to local oscillator signal generation for wireless communication devices.
2. Description of Related Art
Mobile communication has changed the way people communicate and mobile phones have been transformed from a luxury item to an essential part of every day life. The use of mobile phones today is generally dictated by social situations, rather than being hampered by location or technology. While voice connections fulfill the basic need to communicate, and mobile voice connections continue to filter even further into the fabric of every day life, the mobile Internet and moving video, including broadcast video, are the next steps in the mobile communication revolution. The mobile Internet is poised to become a common source of everyday information, and easy, versatile mobile access to this data will be taken for granted. Similarly, video transmissions to handheld user equipment will allow movies and television programs to be viewed on the go.
Third generation (3G) cellular networks have been specifically designed to fulfill many, if not all, of these future demands. As these services grow in popularity and usage, factors such as cost efficient optimization of network capacity and quality of service (QoS) will become even more essential to cellular operators than it is today. These factors may be achieved with careful network planning and operation, improvements in transmission methods, and advances in receiver techniques. To this end, carriers want technologies that will allow them to increase downlink throughput and, in turn, offer advanced QoS capabilities and speeds that rival those delivered by cable modem and/or DSL service providers. In this regard, networks based on Code Division Multiple Access (CDMA) technology or Wideband Code Division Multiple Access (WCDMA) technology may make the delivery of data to end users a more feasible option for today's wireless carriers.
The General Packet Radio Service (GPRS) and Enhanced Data rates for GSM (EDGE) technologies may be utilized for enhancing the data throughput of present second generation (2G) systems such as GSM. The Global System for Mobile telecommunications (GSM) technology may support data rates of up to 14.4 kilobits per second (Kbps), while the GPRS technology may support data rates of up to 115 Kbps by allowing up to 8 data time slots per time division multiple access (TDMA) frame. The GSM technology, by contrast, may allow one data time slot per TDMA frame. The EDGE technology may support data rates of up to 384 Kbps. The EDGE technology may utilizes 8 phase shift keying (8-PSK) modulation for providing higher data rates than those that may be achieved by GPRS technology. The GPRS and EDGE technologies may be referred to as “2.5G” technologies.
The Universal Mobile Telecommunications System (UMTS) technology with theoretical data rates as high as 2 Mbps, is an adaptation of the WCDMA 3G system by GSM. One reason for the high data rates that may be achieved by UMTS technology stems from the 5 MHz WCDMA channel bandwidths versus the 200 KHz GSM channel bandwidths. The High Speed Downlink Packet Access (HSDPA) technology is an Internet Protocol (IP) based service, oriented for data communications, which adapts WCDMA to support data transfer rates on the order of 10 megabits per second (Mbits/s). Developed by the 3G Partnership Project (3GPP) group, the HSDPA technology achieves higher data rates through a plurality of methods.
Where HSDPA is a downlink protocol, High Speed Uplink Packet Access (HSUPA) technology addresses the uplink communication. HSUPA is also specified by the 3GPP group to provide a complement data link to HSDPA. HSUPA also offers broadband IP and is based on software. HSUPA also extends the WCDMA bit rates, but the uplink rates may be less than the downlink rates of HSDPA. Where prior protocols severely limited the uplink connections, HSUPA allows for much higher uplink rates.
Likewise, standards for Digital Terrestrial Television Broadcasting (DTTB) provide for transmission of broadcast video. Three leading DTTB systems are the Advanced Television Systems Committee (ATSC) system, the Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) system, and the Digital Video Broadcasting (DVB) system, which includes terrestrial transmission under Digital Video Broadcasting-Terrestrial (DVB-T) specifications and transmissions to handheld devices under Digital Video Broadcasting-Handheld (DVB-H) specifications. DVB-H is an adaptation of DVB-T to handheld units, in which additional features are implemented to meet specific requirements of handheld units. DVB-H allows downlink channels with high data rates and may be made as enhancements to current mobile wireless networks. DVB-H may use time slicing technology to reduce power consumption of handheld devices.
In order to practice the various communication protocols, a wireless communication device is utilized. For a wireless communication device to participate in wireless communication, it typically includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). The transmitter typically includes a data modulation stage, one or more intermediate frequency (IF) stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The one or more IF stages mix the baseband signals with a local oscillator signal to produce radio frequency (RF) signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
The receiver is coupled to an antenna and typically includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies them. The one or more IF stages mix the amplified RF signals with a local oscillator signal to convert the amplified RF signal into baseband signals or IF signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
One of the components in a typical RF front end is the local oscillator (LO). The LO generates a local oscillator signal that is sent to a mixer to mix the inbound RF signal for down-conversion and/or to mix the outbound signal in the transmitter for up-conversion to a RF signal for transmission. A common technique is to utilize a phase-locked loop (PLL) circuitry for generating the local oscillator signal. PLL circuits implement a closed loop phase or frequency control system that looks at the input and output of the PLL and uses a difference signal to correct for variations at the output. In this manner, a PLL may provide a stable signal at its output, which is then used to provide the LO signal.
When a wireless communication device includes both a transmitter and a receiver resident thereon, separate RF mixers are used on the receive side and the transmit side. The LO, such as the above-mentioned PLL, provides the LO signal to the mixers for up-conversion or down-conversion. With both a transmitter and a receiver, one practice is to use one PLL that generates a LO signal for the transmitter side and a second PLL that generates a LO signal for the receiver side. In some instances, the output of one or both of the PLL(s) is further frequency divided to provide the desired LO frequency for frequency conversion. For example, when a PLL output frequency is selected to be higher than the needed LO frequency, a divider is used to lower the PLL output to a lower frequency of the LO signal. As another example, when multiple LO signals are to be generated at different frequencies, a PLL output may be divided by one or more frequency dividers to produce multiple LO signals at having different LO frequencies. Thus, for these reasons, as well as other, a divider circuitry may be used to divide the PLL output to generate one or more LO signal(s).
A typical practice known in the art is to use current mode logic (CML) circuitry to generate the LO outputs. CML allows the use of differential logic to transmit signals at high speed. Thus, CML or pseudo-CML circuits are used for LO signal generation and dividers at the output of a PLL may also use such CML circuits. When CML circuits are implemented using Complementary Metal-Oxide-Semiconductor (CMOS) technology, resistive loads are typically employed with each of the P and N side of the differential circuit, as well as a common tail current source. The tail current source is typically in a common source leg (for N-type devices) of both the P and N sides. The current source has a high output channel conductance and, therefore, the current source is modulated by its Vds voltage. This may result in excessive variation of total current drain and may introduce switching noise into the power supply line. In reference to the resistive loads, the resistor values are typically small, in order to ensure a low far-spectrum phase noise. However, small resistor values may require high tail current to ensure proper voltage levels for switching.
Furthermore, CML topology is such that the circuit, including the tail current source, is turned on (active state) at all or substantially a good portion of the time. CML topology may also suffer from reduced output swing, so an appropriate converter may be required to restore rail-to-rail switching. All of these actions consume power, which is a major concern for handheld battery operated devices.
In order to circumvent some of the concerns with the use of CML or pseudo-CML circuits for LO signal generation, another technique is warranted. Therefore, a need exists for a technique to generate LO signals without the use of CML topology.